1. Field of the Invention
The present invention relates to protection circuits, and more particularly to diode arrays for protecting differential pairs of digital telephone lines or data processing systems.
2. Discussion of the Related Art
In data processing systems, connectors are often connected to a plurality of lines whose voltage may vary between first and second states corresponding to first and second voltages.
In the following description, it will be assumed that the first voltage is zero and the second voltage is a positive voltage, for example +5 volts. However, it will be clear that, generally, the voltages of these lines may vary between a high voltage and a low voltage, or reference voltage.
A conventional protection circuit for a line of this type is illustrated in FIG. 1. Line L is connected to the reference voltage REF (ground) through a reverse biased diode d1 and is also connected to ground through a forward biased diode d'1 in series with a reverse biased avalanche diode Z whose threshold voltage is close to (slightly higher than) the high voltage on the line. Thus, a negative overvoltage on the line L flows to ground through diode d1 and a positive overvoltage, having a value higher than the nominal voltage of the line, flows through diode d'1 and avalanche diode Z.
This type of protection circuit uses a protection diode having a low avalanche voltage and/or having a high overvoltage absorption capacity, and therefore a large size, while limiting the parasitic capacitance applied to the circuit to be protected due to the presence of diodes d1 and d'1.
In fact, in data processing circuits, numerous lines are connected to a connector and it is therefore necessary to associate therewith numerous protection circuits such as the one of FIG. 1, all having a common avalanche diode. Such a circuit is illustrated in FIG. 2 in which are provided, in association with each Line L1-L8, a diode d1-d8 reverse connected to ground, and a diode d'1-d'8 forward connected to ground through an avalanche diode Z.
An embodiment of a monolithic integration of the circuit of FIG. 2, in which diodes d1-d8 and d'1-d'8 are Schottky diodes, has already been proposed by the applicant in European patent application EP-A-562,982 of Mar. 23, 1993.
The present invention provides an implementation in the form of a monolithic circuit of the circuit of FIG. 2 in which diodes d1-d8 and d'1-d'8 are junction diodes so that the device can ensure protection even against a very high current.
More generally, it will be noted that the circuit of FIG. 2 includes a protection diode Z across the terminals of which a plurality of pairs of diodes is connected. Each pair of diodes includes two diodes oriented so as to have the same polarity as the protection diode. Such a circuit can ensure other applications than the above described application for the formation of a protection diode array. For example, it can be seen with relation to FIG. 4 that such a circuit also corresponds to a self-protected rectifying bridge circuit.